Electro-optical display devices are the object of considerable research efforts. Of the various display systems that have been developed, thin flat-panel display devices utilizing, for example, liquid crystal components are of particular commercial interest.
Compositions characterized as liquid crystals include a wide range of materials. The different electrical and optical properties exhibited by these liquid crystalline materials make possible a number of mechanisms for light modulation. Such mechanisms include phase transitions, dynamic scattering, and field effects, all of which are well known in the art.
Field effect devices are of particular utility. The effect that is commercially most significant at present is the rotation of polarized light by a twisted nematic liquid crystal alignment and the disappearance of this effect when an electric field is applied across the device. Twisted nematic liquid crystal devices typically comprise a suitable liquid crystal composition confined in a cell between two optically transmissive plates, the plates having transparent conductive films affixed to their surfaces facing one another in the device. The alignment of the surface layers of the liquid crystal in the "off" state (i.e., the "open" state in most liquid crystal displays) of the device is determined by the interaction of the liquid crystal composition with the confining surfaces of the display device. The orientation of the surface layers of the liquid crystal is propagated throughout the bulk of the composition.
To effect orientation of a confined liquid crystal, the internal surfaces of the conductive plates of a sandwich display device can be prepared by unidirectionally rubbing the surfaces prior to fabrication of the device. The liquid crystal molecules immediately adjacent each rubbed surface tend to orient themselves in the same direction as the rubbing. By arranging the opposing conducting plates with the axis of the rubbed surface at, for example, right angles to each other, the liquid crystal molecules at points intermediate the two plates will orient themselves to a degree which is a function of the distance from the two plates. Accordingly, in this example, the liquid crystal will align itself in a continuous spiral path that twists through the 90.degree. angle between the opposing plates.
If the light-rotating liquid crystal "sandwich" is mounted between two crossed light polarizer elements, then illuminated by an internal light component, polarized light will pass into the liquid crystal cell (i.e., in its "open" state) and be rotated through a 90.degree. angle as it is transmitted through the twisted nematic crystal composition from one surface of the cell to the other. Due to the 90.degree. light rotation effected by the twist of the liquid crystal, the polarized illumination light will be set to pass through the second crossed polarizer mounted downstream.
When an electric field is applied across the liquid crystal composition between the two conductive plates, the twisted orientation of the liquid crystal is obliterated as the molecules align themselves with the applied field. As the liquid crystal is untwisted, the polarized illumination light entering the device through the first polarizer will no longer be rotated 90.degree. as it is transmitted through the liquid crystal. Therefore, the non-rotated light will be unable to pass through the second polarizer which is set correspondingly crossed to the first polarizer (i.e., the "closed" state). Selective application of voltages across discrete segments of the liquid crystal device can readily accomplish patterns of bright areas (no applied electric field) and dark areas (applied electric field).
In devices employing internally-illuminated LCD displays (e.g., wrist watches, calculators, personal digital assistants, cellular telephone displays, and laptop computers), backlighting and edge-lighting--when used--consume vigorously the power supply of said displays. To reduce reliance on such internal lighting systems, it has been found that displays adequately viewable under good ambient light can be provided by employing in said displays a reflective holographic diffuser. See, International Patent Application No. PCT/US96/06852 (Publication No. WO 96/37805)(Wenyon). See also International Patent Application No. PCT/US94/11818 (Publication No. WO 95/12826)(Chen et al.). In this role, the reflective holographic diffuser provides a bright illuminating background by the holographic modulation of available ambient light, rather than by energy-depleting electroilluminescence.
While holographically-illuminated liquid crystal displays accomplish good results in several applications, considerations of the possibility of use in poor ambient light conditions, has made desirable for certain display configurations the incorporation or retention of supplementary internal light sources. A problem arises because highly reflective layers of metal (e.g., aluminum) are employed in the manufacture of certain varieties of reflective holographic diffusers. When the holographic reflective diffuser in an LCD display is placed proximate the backside of the display's image-providing element, the highly reflective layer can block from a viewer the light propagated from the internal lighting source.